As is well-known, electromagnetic waves can transport and deliver energy to an object or load. Microwave applicators using electromagnetic waves in a frequency range of 300 MHz to 300 GHz generally include a microwave energy source, a microwave containment chamber, and a microwave feed structure coupling the energy source to the microwave containment chamber.
A preferred microwave energy source for the present invention is a magnetron operating at 2450 MHz, although it is to be understood that since 915 MHz is an approved microwave cooking and heating frequency, the present invention is adaptable to operation at 915 MHz, and any other microwave frequency desired, according to the teachings hereof.
The volumetric space within a microwave containment chamber is a cavity in which the load (the object or substance to be heated) is placed.
One of the most significant problems with prior art microwave applicators is uneven temperature distribution in the load. Uneven heating is mainly due to three causes: mode-related hot and cold spots, edge overheating, and underside underheating.
Each mode has a respective vertical guide wavelength .lambda..sub.g. When modes in a system can be excited so that the modes do not couple to each other even if the system is lossy, the modes are called orthogonal modes.
In the prior art, hot and cold spots occurred because of the uneven energy distribution particular to the modes in the cavity of the applicator. The electric and magnetic field configuration of a mode is dependent on the operating frequency and the dimensions of the cavity.
There are two distinct classes of modes, transverse magnetic (TM) modes and transverse electric (TE) modes. TE modes have no electric or E field component in a direction of propagation, while TM modes have no magnetic or H field component in the direction of propagation.
TE and TM modes are labelled as TE.sub.mn and TM.sub.mn. For a rectangular waveguide, the subscripts indicate the number of half-period variations of a mainly transverse field vector along paths parallel to a wide wall (m) and a narrow wall (n). In a rectangular coordinate system, the m and n subscripts conventionally refer to the x and y axes, with propagation occurring along the z axis.
In a cylindrical cavity it is convenient to use a polar coordinate system. In the present invention, the direction of propagation is along a z axis parallel to the longitudinal cylindrical axis of the cylindrical cavity. In a circular cross-section waveguide or cavity, i.e., one having a generally circular wall concentric to the direction of propagation of microwave energy in the waveguide or cavity, the subscript or index m indicates the number of full-period variations of a transverse field vector along a circular path concentric with the wall. Subscript or index n indicates the number of reversals plus one of the same vector along a radial path in the cavity.
The traditional solutions to avoid mode-related hot and cold spots were either to use a mechanical device (e.g., a turntable) to move the load in relation to the cavity during heating or to use a "mode stirrer" to continually alter the mode patterns within the cavity. Mode stirrers are typically fan-shaped mechanically rotating structures with metal blades placed either inside the cavity or in a separate open feedbox adjacent the cavity. Some designs have attempted to reduce hot and cold spots by using devices such as multiple feed arrangements or rotating antennae.
There continues to exist a need for an efficient microwave applicator that offers convenient and reliable time-averaged uniformity of microwave heating.
Edge overheating (hot spots on the edges of the load) occurs due to the direct coupling of an E field component parallel to an edge of the load, and becomes more significant when the load has a high permittivity.
In most microwave ovens, the loads are generally dielectrics, such as food, with a rather high relative permittivity. The microwave modes interact with the high .epsilon. load to transfer energy into the load .epsilon..
It is important to understand that the H field intensity in the load and the heating pattern are directly related. Maxwell's equations reveal that energy absorption of the load is generally through the electric E field. Prior art applicators attempt to maximize E and H field intensity to maximize energy transfer and minimize cooking time. However, in so doing, the prior art applicators increase edge overheating, and the possibility of microwave leakage.
Another microwave heating problem is low or insufficient "underside" heating of a flat load. Since not much power penetrates through a flat load, the underside of a flat horizontal load is usually poorly and unevenly heated. Absent a microwave feed below the load, "underside" heating requires the load to not extend over the whole cross section of the cavity.